building a transmitter

Hello iam wondering if anyone knows how to build a long range f telephone bug that will transmitt on the fm frequencys for distance of 5 miles. I dont plan on using this thing for any crminal acts jus want to pull a prank on some people! would it even be possible t make something like this? If that is not possible is there any wa you could increase the distance on an portable phone to thi distance

Reply to
ilox
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A transmitter for that kind of power must be licensed and/or type-accepted.

Do you mean broadcast FM, or just any band with FM modulation?

Reply to
Don Stauffer

Do you have a line of sight from transmitter to receiver? Can you use directional antennas at both locations? Do you mind if you break a few laws? Does it have to be FM? How long do you want to be transmitting? Will you use batteries / how much power is available? Does it have to be concealed? How small?

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perferably it would be better if it was powered right off th telephone line, but i dont think there would be enough power.Th phone company relays would trip at about 40 milliamps. it doesn have to be concealed, perferably not to big but size isnt a rea issue there is no line of site from transmitter to reciever, it coul be fm or whatever works best, no i dont care to break a few laws a iam not using this for any illeagal purpose or to harm anyone

Reply to
ilox

is there any way of using a wireless rf modem to tapp a phone lin and then bring the information to a reciever at another locatio through the rf. some of those modems have ranges of up to 30 miles o more

Reply to
ilox

i dont know anything about directional attenas, to power it with 12 would be good though right off the secondary of a transformer. would want it to transmitt whenever a call is made. small version exsist but do not have a long range. maybe 400 feet if that

Reply to
ilox

No.

How many times do you need to be told none of your ridiculous phone tap ideas will work?

Reply to
Don Bowey

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has a 5 watt transmitter more complicated than you want I'm sure, but the output stage should be adaptable to another oscillator and driver stage.

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has a 14 dB VHF power amp that can work over 2-20 watts

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Simple 150-300 mw transmitter that could drive the VHF pa to several watts

Takes more power than any phone line will supply. You'd still need a damn good antenna.

It doesn't matter what your motive is, bugging a telephone is illegal, and putting it out over any transmitter let, alone an illegal transmitter won't help.

Or you could just buy a cheap CB radio and connect it to the phone.

Or get a cell phone and subscribe to a "walkie talkie" phone service and plant a phone at the point of interest - at least that might stay below the radar longer than another method.

This prank of yours is going to cost you some money even if you're not caught - fines likely if you are caught. In the middle of Montana you might get away with it, in the middle of Manhattan your chances of being caught are greater.

Tape recorder and VOX is easier if you don't need wireless. As long as you're not picky about how illegal it is, perhaps you can tap the signal on the way to the central office while it is still analog on copper.

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You asked how many times do you have to be told none of you ridiculous phone tap ideas wont work? The answear is ONCE just by yo

Reply to
ilox

you have made the point that it is illegal, however in canada th consequences are a bit less severe. none the less it is still illega here, but how would you go about the idea with the cb radio? a bit o details would be good on the connection of this equipmen

Reply to
ilox

Chek the links at

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There are low power "telephone bug" devices that are powered through telephone line.

Even owning such transmitter device is illegal in many countries.

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Tomi Engdahl (http://www.iki.fi/then/)
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Reply to
Tomi Holger Engdahl

I wouldn't go about it, but if I did, I'd start with a way to turn the transmitter on and off. That's easily available on a CB because most use a push to talk switch that is accessible from the microphone jack.

Then I'd figure out how to key the transmitter whenever the phone was "off hook." Google for things like "off hook" (the phrase) along with terms: telephone, circuit, schematic.

There's tons of stuff - just reading the returned phrases I'd say you'd need a voltage sensitive relay that would detect 50 volts. These used to be common, today you'd probably use a comparator to sense the voltage change - doubtless the circuit is already out there.

Then I'd want an amplifier for the audio so I could capacitively couple audio from the phone line without disturbing the operation of the phone. This could also be done with a 600 ohm or other audio transformer - So there might be some representative circuits for that too.

An amp gives you more control adjusting the volume for the transmitter and is harder to detect when looking for bugs (assuming the people you are bugging are knowledgeable). Less or no chance of clicks on the line when connecting or disconnecting the device (if that's a concern).

Downside might be a ground loop - but you'd have to deal with that when and if it became a problem.

I'd protect my amp from the 90 volts of AC that is across the lines when the bell rings (assuming its the same in Canada).

Output of the amp goes into the microphone input to the CB.

Another way would be to use a VOX (voice operated switch). Keys the transmitter whenever audio was present. but you said you want to monitor the telephone, not the whole room and a voltage sensitive relay is less complicated.

CB's may be designed with some duty cycle in mind. Typically one transmits for a time then listens for a time . . . If you're just going to key the mic when the phone is off hook, there may be a need for a bigger heatsink on the output transistors. They are only five watts - so that may not be a concern - I don't use CB or know much about it these days.

All you want to know is probably already on the web

search terms: relay schematic recorder "off hook"

returned these (among over 600 hits) that look like what you could use. I know you don't want to "record" the conversations - but people that do, also have to figure out how to turn the recorder (transmitter) on and off, and amplify signals, and deal with ring voltages, etc..

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Bear in mind you still need an antenna on the CB - particularly the transmit side or you risk burning out the output stages. The antenna has to match the frequency you want to transmit at. One of those sites I quoted for VHF transmitters shows a neat J-pole antenna and has a java script program for calculating the lengths to match the impedance of the transmission line - or you can just use what CBr's use (buy a CB antenna).

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is this amp really that important will it increase distance o transmission? where do you hook it up, input to the telephone line and where would you wire in the voltage sensitive relay, in series o paralell, before or after the amp? and the heatsink on outpu transisters iam not even going to ask about those i have never eve heard of them

Reply to
ilox

The importance of the amp is relative. It gives you greater control of the audio level; it isn't necessary and you could just use a coupling transformer. I would probably choose to use it, even if I also used a coupling transformer - the transformer would eliminate the possibility of a ground loop.

A fancier idea for an amplifier would be a "compressor." They boost all signals then attenuate the stronger ones so all audio is at a similar level - that does make a difference in the intelligibility of AM transmissions at a distance. 5 miles should be no problem for a CB. Advantage there: a whisper is as intelligible as a shout.

The voltage sensitive relay goes across the two wires that carry the phone signals. That voltage is around 50 when "on hook" and 10 or less "off hook." It is in parallel with your audio pickup.

Sounds like you're out of your element on this project. Transistors get hot, when asked to control power especially in their linear range (in between strictly on or strictly off).

The cases are usually attached to a chunk of aluminum to increase the surface area and allow the heat to transfer to the environment rather than build up in the transistor. The chunk of aluminum (or other good conductor of heat) is called the heatsink. It can be part of the chassis - or a specially made piece that is thermally bonded to the output transistor (and sometimes driver transistors as well). Those black things with fins on the top of the CPU in your computer - heatsink(s).

Transistors can burn out.

You exacerbate the problem if your load (antenna and transmission line) doesn't match the output impedance of the CB (usually 50 ohms).

The power that is reflected back down the line (the power that doesn't go out the antenna) is dissipated in the output stages as heat and is potentially destructive to the output stages. (although I'm sure CB manufacturers already take that into account when they design their gear since the average consumer doesn't understand that).

Your application calls for turning the transmitter on for a long period, so you have to be concerned with heat.

You remember that TV antenna wire that is flat with the two conductors on opposite sides of the plastic center (called "twin lead")? Well that stuff has an impedance of 300 ohms, which is what many TV antennas match. But the cable input is coaxial on most TVs and cable systems. Coax cable for TV is 75 ohms. You add something called a "balun" (transformer) that converts the impedance from 300 to 75 - and prevents reflections on the transmission line. Not destructive to TV receivers, but it is destructive to TV signals - you see a double image or ghost displaced a little bit from the real image.

In a transmitter the power level is higher and that reflected power is lost as heat. Cuts down your output signal as well. Transmission lines are whole field of knowledge . . . When CB'rs or hams talk about SWR (standing wave ratios) they are talking about the ratio of power generated that actually makes it out into the airwaves.

Tesla coil builders have a intimate understanding of SWR - since the reflected power can make big destructive noise, light and smoke. Same thing with high power transmitters. I worked on 30 to 100+ kilowatt rigs - a VSWR problem could turn rivets into a hot shower of sparks in no time at all or arc spectacularly before some safety device shut things down.

I'm becoming a lot less worried that you might actually do this . . . It is sounding like you have a lot to learn before you can pull this off.

You know illegality aside - how about concern for your fellow man? Telephones can be used to convey some damning or embarrassing stuff. There are actually people out there that don't want the Western Hemisphere to know they have a sexually transmitted disease. How would you like it if someone monitored your calls?

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the likelyhood of this project going ahead is decreasing due to m lack of knowledge with all this apparatus. I'am a third yea electrical apprentice and have knowledge about basic wiring but no electronics and radio equipment. One question though if you put th voltage sensitive relay across the 2 wires you are effectively tyin up the line it would be the same as lifting the phone off the hoo wouldnt it, just in a different location where the relay is placed this would effectively make it seem as if the phone line is in use a long as the relay is attached to the two wires? hence i dont thin this would work would it

Reply to
ilox

ummmm what's the project? What apparatus do you have, or want to create?

It's always proper etiquette to include the pertinent parts of previous posts to maintain continuity.

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Reply to
DaveM

It sure would make the line appear off hook if the impedance of the relay were low enough. That's why I mention they use comparators these days.

"Back in the old days," we had relays that would operate on less than

50 microamps for just such purposes. They would be expensive to manufacture today, or more so than a solid state circuit and el-cheapo relay. (I'm not that old, but the stuff was more common when I was a kid)

The relays had a variety of uses in tube circuits where the current was low and impedance had to be high. "balanced armature, design" with adjustments for contact spacing and spring tension, etc.. They still show up at ham fests, military surplus outlets, and in antique equipment. Place called "Fair Radio Sales" had a few recently - mail order and on line.

Almost everything that is done in electronics today with semiconductors was done in the past - differently.

It may still be possible to find a 120 VAC relay and operate that at

50 VDC to good effect - I don't have one handy so can't measure the resistance. AC relays operate at lower voltage when DC is used. A 120 AC relay might have a coil that would be in the 5K-12K ohm range. That would impose a current of 10 to 4 milliamps on the line.

A typical telephone is about 680 ohms "off hook," so it seems plausible that a 120 AC relay might work without showing up as off hook to the central office.

Mind you - this would be a "sensitive" relay, not something used as a power contactor designed to switch high currents. It would take experimentation, but logically, it ought to work . . .

Relays can be "tweaked" Bend the normally closed contact closer to the pole and you lower the operating voltage, because the armature is closer to the pole piece. Stretch the spring and you do the same thing - less force to overcome (the current carrying capacity of the contacts may drop and or voltage ratings of the contacts).

Magnetic force drops with the cube of the distance - so a small difference in armature spacing is a large difference in operating voltage for a relay.

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If you get interested in early telephone equipment:

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Cyclopedia of Telephony & Telegraphy Vol. 1 A General Reference Work on Telephony Whole book is one html file. Has drawings of all the early telephone equipment: receivers, transmitters, induction and loading coils, electromechanical repeaters, magnetos, etc..

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don't know what your trying to do but you can get voltage sensitive relays designed to trip at set level that you wish for. the actual load they exert on the line is normally so minimal that it does not effect any other operating equipment on that line. the relay of course normally needs to be powered up from another source ie 120 v control source ect.. there are also relays designed to use the line if self as it's actuating source and perform a hysteresis and are Set able, this of course is a mechanical approach an does consume what it needs from the line to energize the coil.

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Reply to
Jamie

So basically all i need is a relay to have 680 ohms or more o impedance and it wont make the line appear off hook? thes comparators you speak of i have searched the net but cant seem t find out much about them. do these things have or act like relay where they switch the postion of contacts? what do you do exactly o did you do that you know so much about all this stuff

Reply to
ilox

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